ulterior

Ulterior, ulterius is a third declension comparative whose positive has vanished. Nothing survives of an "ulter"; the missing degree is supplied by the preposition and adverb ultra, "beyond", and the superlative is ultimus, "furthest, last". Comparatives decline as consonant stems, so ablative singular ulteriore, genitive plural ulteriorum, and the neuter plural is ulteriora.

Roman administration fixes it geographically, always seen from Rome: Hispania Ulterior and Hispania Citerior, the further and the nearer Spain, and Gallia Ulterior beyond the Alps. The neuter singular ulterius doubles as the comparative adverb, "further, any longer", and that is how the corpus mostly has it. English "ulterior motive" keeps the idea of something lying beyond what is on show, which is a fair extension rather than a mistranslation.